1. Technical Field
The present application concerns the blend of plastics with wood particles or other plant particles or the like. As far in the following only wood or only wood particles are mentioned, then this includes other plants and plant particles.
2. Background Information
Background information is for informational purposes only and does not necessarily admit that subsequently mentioned information and publications are prior art.
Plastics are used in manifold applications:                molded parts, work pieces, blocks, boards, films, webs, coatings, tubes, hoses, bars, rods, profiles, bands, strings, wires, bristles, grids, adhesives, lacquers, glues, pastes, mastics, binders, colours, plasters, spittle, casting compounds, sealing masses, melt and coating materials, gels, filaments, brins, threads, silks, strands, mats, fleeces, fabrics.        
Plastics can consist of monomers and/or polymers. Often it is a blend, whereas also blends of other materials than plastics are found. This applies especially for the production of plastic foam.
For the compounding of plastics the use of extruders is commonly used.
By means of an extruder the used material can be very well melted, mixed and homogenized and dispersed respectively.
Moreover, the used material can simultaneously be heated or cooled. For heating or cooling there are heating and cooling lines in the extruder shell and, where necessary, in the spindles. In addition, the deformation of the used material performed by the extruder spindles causes a considerable heating.
Very difficult materials can be blended in the extruder. Wood and plastics are belonging to the materials difficult to blend. Thereby the wood is supplied in small particles into the extruder and there wrapped with plastic. In order to provoke the wrapping the plastic must be plasticised. This happens under corresponding heating and under pressure. However, the heat is only very badly transported by the wood. Moreover, the wood is very porous. As per an older proposal there might be the danger that the plastic will be pressed disproportionately into the pores and will be unevenly distributed and no all-sided wrapping of the wood particles will take place.
The wood-plastic blend is also described as wood substitute.
Wood is used in several fields of application. Furniture, interior fittings, trade fair construction, even automotive linings.
With regard to the furniture you have to distinguish between furniture where wood is only construction material and furniture where wood is used as visible surface.
For upholstered furniture wood is mostly used as construction material.
Generally all upholsteries have a solid substructure, e.g. a frame, also called rack. In and at the rack there are optionally mounted seats, backrests and armrests as well as feet. Optionally the rack forms also directly the seat, backrests and armrests or vice versa. The material out of which the rack is produced is the construction material.
Traditionally the frames for upholsteries are produced of wood. Boards and sticks are used. Wood has the advantage that the single parts can be connected with each other as well as with the upholstery and the upholstery cover by the so-called stapling. For the stapling u-shaped fasteners are used. The fastener replaces the bullen-nails of the former days.
Other furniture show the wood as visible, decoration or ornament areas respectively.
To some extent only specific rare kinds of wood fulfil the requested requirements for this. Rare lumber is relatively expensive. Since long it is therefore state of the technology to cut veneers out of rare wood which forms the visible surface. Veneers comprise thin, filmy woodcuts. A considerable waste occurs when the veneers shall be processed like the wood is grown.
By the blend of wood and plastic an advantageous wood substitute occurs.
Instead of wood also other comminuted plants can be used. Optionally an extrudate is generated from plastic and wood and other additives, which is foamed or unfoamed. At low wood proportion in the extrudate you speak of an armouring or about the use of wood as filler. In the USA you speak of a wood substitute if the wood proportion in the blend has reached a weight proportion of 50%. For example, boards made of the blend of wood and plastic with the indicated blending proportion are common practice in the USA. The US-market accepts this. An example for the blends usual in the USA shows the WO 02/103113A.
The European market only accepts a wood substitute in the visible area only, if the appearance of the material is essentially similar to wood. Thereby 70% and more of wood are required.
However, an enduring outdoor application free of defects could up to now not be reached with such a material.
For the use of other plant components, e.g. straw instead of wood is similar with the wood. To some extent the plant part have a lower stability than wood. As far as the stability is concerned the blend must be changed accordingly. As further plant components also grain grist and comminuted hay can be used.
As per an older proposal the plastic proportion of the extrudate shall be reduced as far as possible. Thereby wood proportions at the extrudate of 60 to 95 wght % referred to the blend are aimed for. The optimal plastic proportion shall be added against the blend proportions, the processing and the intended characteristic values of the blend.
As per the older proposal the dimensions of the plant particles are destined by the traversing of the particles in the extruder when using traditional extruders. Thereby the traversing is determined by the clearance of the moving extruder parts in the extruder and by the opening width of the die gap. The traversing can be determined by means of the known machine data. Then the dimensions of the plant particles, admissible for the respective extruder, can be determined. For safety reasons, you can choose smaller dimensions of the plant particles than the above described traversing.
Alternatively, as per the older proposal the traversing of the extruder can be adjusted to the requested particle size. This is done by means of the new construction of the extruder or the exchange of construction parts, e.g. the exchange of the extruder screw, the die or the liner in the extruder housing. Modern extruders have a liner in the extruder housing, in which the screws are rotating.
As plastics all extrudable plastics can be considered, especially Polyethylene (PE), Polystyrene (PS), Polyurethane (PU) and Polypropylene (PP). Commonly desired is the use of PE.
The bonding agents can compensate a lack of adhesion of plastics with plant particles and wood respectively.
Bonding agents generate molecular bridges at the interfaces between the materials to be connected, here the plastic.
As far as also other fillers or armouring materials/reinforcing agents like e.g. glass fibers are used, the bonding agents can also have the task to increase the adhesion to these other materials. Bonding agents can be VC (vinyl chloride)-copolymers, phenolic resins, cautchouc derivates or acrylic resins without or with PF (phenol-formaldehyde)- and EP(epoxy)-resins.
As bonding agents are known generally also EVA (ethylene-vinyl acetate-copolymers; It might also already be sufficient to make the plant particles water-repellent, i.e. to decrease the surface tension.
Traditionally the plastic is supplied as granulate with plant particles and additives into the extruder. Optionally the granulates contain already a blend of plastic and additives.
By means of drying the humidity can be reduced to every degree required for the extrusion. Not always a minimal degree of humidity is required, but optionally a destined moisture content by which you can influence the extrusion process and/or the condition of the extrudate.
So far, twin-screw extruders have been regarded as especially favourable because these extruders have:                a very high feed effect        a very good pressure build-up.        
Advantageously, extruders from different elements and section of different kind of construction respectively can be assembled. Therefore it is possible to insert for the plasticising zone an element in the construction type of a twin-screw extruder, being advantageous there, and to combine this element in other extruder zones with other kinds of construction, having there advantages. Thus, a single-screw extruder section can be used in the feeding zone with which the pressure build-up can be presented advantageously. For the blend and homogenization other sections will be better.
Under pressure and temperature the plastic is melting. In the further step of the blend used through the extruder the blend will be homogenized. For the homogenization zone and the dispersion zone it is of advantage to us there extruder elements having a high mixing capacity. These are for example elements in the design of a planetary roller extruder. This element has simultaneously a high cooling effect by which the processing temperature can be controlled very exactly. Depending on type of plant and type of wood respectively and depending on the plastic an optimal processing temperature for the melt is resulting. The temperature can e.g. amount up to 200° C. at a max. dwell time (temperature loading time) of 15 min. At a shorter duration the max. temperature can be higher than at a longer duration.
At the further extrusion a differentiation must be made between the extrusion of unfoamed plastic and the extrusion of foamed plastic.
The residual moisture can serve as lubricant. The residual moisture can also enter into chemical reaction. For example the moisture can enter into reaction with the lignin of the plant particles. Surprisingly an ameliorated adhesion between wood and plastic can be caused by this. The adequate content of moisture/lignin can be tested by variation of the moisture and variation of the lignin content. Lignin is also available in customary liquid form.
In case no water is desired at the extrusion, this can be counteracted by drying of the plant particles before the extrusion. In addition or instead of the drying the moisture can be eliminated after the vaporization in the extruder by means of the degassing. The degassing can occur immediately after the vaporization. This takes place regularly in the plasticization zone. There the necessary heating takes place. This heating results from the forming works during plasticization and, if necessary, by supply of heat. The heat supply can take place e.g. via a heating-cooling system in the extruder housing.
The degassing takes place, if requested, latest directly in front of the extrusion die.
The degassing requires that the melt pressure will be reduced.
Known is also the degassing by use of two extruders in tandem arrangement at the transfer between the two extruders.
The degassing can also take place in one extruder. The necessary pressure reduction in the melt can be carried out by different measures, e.g. by change of the pitch of the screw in the extruder.
For the degassing the melt can also be taken-off out of the extruder and led via a degassing device and returned again into the extruder.
By this, the pressure control can be essentially made easier by interconnection of a melt pump. This is valid also for the pressure directly in front of the extrusion die (tool). Here the pressure can be kept additionally constant by means of the melt pump and thus the quality of the extrudate can be improved.
The higher the wood proportion in the blend, the bigger is the danger that the material will be no longer suitable for an outdoor application. The known materials decompose after some time due to steady changes of freeze/thaw. A further disadvantage of the known materials a disproportionate swelling which easily leads to bigger damages or for which oversized expansion joint must be considered.
Moreover, the wear of the extrusion line will be increased with an increasing wood proportion.
The wear reduces the efficiency of the line.